In recent years, silicon nitride masks have become a sought-after expedient in the fabrication of integrated circuits. Originally, the art applied masking layers comprising silicon nitride directly onto silicon substrates. This gave rise to problems associated with high stresses created on the underlying silicon substrate by the silicon nitride-silicon interface. Such stresses were found in many cases to produce dislocations in the silicon substrate which appear to result in undesirable leakage current, pipes and otherwise adversely affect the electrical characteristics of the interface. In order to minimize such interface stresses with silicon nitride layers, it has become the practice in the art to form a thin layer of silicon dioxide between the silicon substrate and the silicon nitride layer. While this approach has been relatively effective in the cases where this silicon dioxide-silicon nitride composite is utilized only for passivation, problems have arisen where these silicon dioxide-silicon nitride composites have been utilized as masks, and, particularly, when utilized as masks against thermal oxidation. During such thermal oxidation, there is a substantial additional lateral penetration of silicon oxide from the thermal oxidation beneath the silicon nitride. This lateral penetration is greatest at the mask-substrate interface to provide a laterally sloping structure known and recognized in the prior art as the undesirable bird's beak which warps and lifts the silicon nitride layer over it. If a recess has been etched in the silicon being oxidized through the mask, then, the beak is combined with a bump extending in the order of 4,000 to 5,000 A above the silicon-mask interface to produce the undesirable structure known and recognized in the prior art as the bird's head.
The publications, "Local Oxidation of Silicon; New Technological Aspects," by J. A. Appels et al, Phillips Research Report 26, pp. 157 - 165, June 1971, and "Selective Oxidation of Silicon and Its Device Application," E. Kooi et al, Semiconductor Silicon 1973, published by the Electrochemical Society, Edited by H. R. Huff and R. R. Burgess, pp. 860 - 879, are representative of the recognition in the prior art of the stress problems associated with directly applied silicon nitride masking layers as well as the bird's beak and bird's head problems associated with silicon dioxide-silicon nitride composite masks.
The bird's beak and bird's head problems are particularly significant when silicon dioxide-silicon nitride composite masks are used in the formation of recessed silicon dioxide to be used for dielectric isolation. In such recessed oxide formation techniques, the silicon dioxide-silicon nitride composite masks are first used as an etch barrier while recesses are etched through the mask openings in the silicon substrate. These recesses are subsequently subjected to the previously described thermal oxidation to form recessed silicon dioxide regions providing dielectric isolation extending into the silicon substrate from the surface. Such recessed silicon dioxide regions would be most desirably coplanar with the remainder of the silicon surface. However, because of the bird's head, an undesirable bump in the order of from 4,000 to 5,000 A in height is present at the surface, but even more significantly, as a result of the bird's beak, a lateral junction or edge of the recessed silicon dioxide isolation region is very vaguely defined. With any recessed oxide isolation it is highly desirable that the lateral edges of the recessed silicon dioxide be substantially vertical, i.e., perpendicular to the semiconductor substrate surface. Instead, as a result of the bird's beak, the edges of the recessed silicon dioxide are gradually sloped with respect to the silicon surface, being at an angle which varies from 15.degree. to 30.degree. with respect to the surface instead of the desirable 90.degree. angle.
Because of this gradual lateral junction in the recessed silicon dioxide, the recessed area does not definitely define abutting regions introduced by either diffusion or ion implantation, particularly shallow abutting regions. In the case of such shallow abutting regions, there is a distinct possibility that during subsequent etching steps part of the bird's beak at the surface will be etched away to provide an undesirable exposure of the P-N or other junction of the abutting shallow region. However, even with deeper regions formed by diffusion, the indefiniteness of the lateral junction of the abutting recessed silicon dioxide region renders it difficult to control lateral geometries of introduced region, and therefore imposes the need for wider tolerances of lateral dimension in the integrated circuit layout.
In addition, during the formation of contact openings to the substrate, there is a distinct possibility that portions of the recessed silicon dioxide near the surface will be etched away to expose silicon portions that are curved or slanted rather than planar. In this connection, in methods and structures such as that defined in our patent, U.S. Pat. No. 3,858,231, where the recessed silicon dioxide is used to define the area of a Schottky barrier contact with the silicon substrate, it is particularly desirable that the recessed oxide abutting the contact opening has lateral sides substantially perpendicular to the semiconductor surface; otherwise, i.e., in the case of a bird's beak, the Schottky barrier contact would be formed with a silicon substrate having a sloped portion. In order to ensure complete elimination of the edge effect problems described in said patent, it is most desirable that the silicon surface to which the Schottky barrier contact is made is substantially flat and coplanar with the recessed silicon dioxide region.
In addition to the bird's beak and bird's head problems described above, the art has also been aware of a lesser problem with silicon dioxide-silicon nitride composite masks involving the undercutting of the silicon dioxide during the etching of openings in the bottom silicon dioxide layer corresponding to the openings previously formed through the silicon nitride layer. Accordingly, the dimensions of the openings in the bottom layer are less definite, and changes in the silicon substrate due to processing through the mask, e.g., diffusion or etching become more difficult to define.